The desire to have high capacity information conduits reaching residential customer premises has promoted intense interest in broadband transmission over copper cable, wire, wireless, and optical fiber media. Fiber-to-the-home, in which optical fiber transport is used over the entire path, is appealing for its large information capacity. Various techniques are available for separating different services for transmission over the same lines, for example the transmitted signals may be time, wavelength, or sub-carrier frequency multiplexed.
Passive optical networks (PONs) are architectures in which there are no intervening active components between the host digital terminal or central office (CO) and customer premises. PONs are desirably installed into remote units, such as homes, to provide data such as video and audio and the like over a fiber.
In other words, PONs require no active components for directing optical signals between the CO and a network subscriber's terminal equipment. Passive optical networks, therefore, require no power or processing in the field to direct optically encoded information to its destination. Typically, a PON includes a first fiber star formed as a plurality of optical paths extending from the CO to a remote node. Downstream optical signals are transmitted from the CO to the remote node. where the signal is passively split and distributed to one of a plurality of units of network subscriber equipment. The network units may transmit optically encoded signals upstream to the remote node to form a multiplexed signal for distribution to the CO. Lasers are generally used to generate light used to form the transmitted light signals.
A standard PON model is shown in FIG. 1, and consists of a first fiber star 1, typically a plurality of optical fibers 2 extending from a central office 4, to one of a plurality of remote nodes 6, i.e., RN.sub.1, RN.sub.2, . . . RN.sub.N. Downstream signals are transmitted from the CO 4 towards the remote node for further distribution. At the remote nodes, light is passively split and distributed via a plurality of optical fibers 8 (a second star) to a plurality of optical network units (ONUs) 10, i.e., ONU.sub.1, ONU.sub.2, . . . ONU.sub.N. The ONUs 10 provide service to one or more end users wherein each downstream optical signal is received and electronically distributed to end users. The ONUs 10 may transmit upstream signals which are combined at the remote node. Each remote node 6 (or passive star) passively combines transmissions from the ONUs 10 onto a single optical fiber 2 for distribution to the CO.
Two passive optical network architectures are a telephony over passive optical network (TPON) and a wavelength division multiplexing passive optical network (WDM PON). In a TPON architecture, a CO broadcasts a downstream optical signal to all ONUs using time division multiplexing (TDM) protocol. A laser with a common wavelength band, requiring synchronization, may also be used. TDM typically includes a frame of information subdivided into time slots assigned to individual ONUs.
Wavelength division multiplexing (WDM) is a technology in which multiple wavelengths share the same optical fiber in order to increase the capacity and configurability of networks. WDM generally increases optical system capacity by simultaneously transmitting data on several optical carrier signals at different wavelengths. The total system capacity is increased by a factor equal to the number of different wavelength channels. WDM PONs utilize an architecture within which each ONU or subscriber is assigned a unique wavelength by the central office. Signals destined for each remote node (and ultimately, each optical network unit) are created by modulating light at N distinct wavelengths at the CO. The modulated light is multiplexed onto a fiber directed to the remote node. The downstream signals are split and distributed to the ONU as a function of wavelength within a wavelength division demultiplexer at the remote node. In the upstream transmission direction (optical network unit to remote node), the light is transmitted at assigned wavelengths, typically by a laser.
Compared to TDM PONs, WDM PONs have the advantage that they do not broadcast individual subscribers' data to all premises. As a result, privacy is enhanced and the electronics in the ONU need only operate at the subscriber's data rate. However, upstream transmission through a wavelength routing device can be difficult. Temperature-controlled single-frequency lasers at each home are impractical. Spectral slicing of light emitting diodes and the use of modulators combined with an optical loopback have also been used. Another approach is to use low-cost, uncooled Fabry-Perot lasers at the home and combine them at the remote node with a passive splitter. This approach is costly because it requires extra fiber and an extra passive component (a WDM splitter) at the remote node, unless a single passive device can accomplish both downstream wavelength routing and upstream power combining. Thus, wavelength division multiplexing, with different services on different wavelengths, requires additional optical transmitters and receivers to be installed wherever an expansion of services and additional channels is required. Each remote unit is assigned a different frequency using a wavelength router. However, due to a number of technical problems, this WDM system is not commercially viable for mass market applications like fiber distribution to the home. One such problem is the small number of channels currently accommodated. Present multichannel laser diodes are very difficult to fabricate with acceptable yield even with as few as eight channels. In addition, passive WDM splitters currently available have a large temperature variation of their passband channels, thereby requiring a continuous tunability in the multichannel sources that has not yet been achieved.
Another type of PON is a power splitting PON (PSPON) which is used with a single-wavelength TDM-encoded transmitter in the central office. One fiber from the CO is directed into a standard power splitter instead of a wavelength splitter. Thus, each remote unit gets a fraction of the total power. This is a time domain multiplexing protocol in which all remote units get the same data, but only the data intended for the particular remote unit is retrieved by the remote unit using, an ID code, for example. These wavelength-independent PSPONs utilize time division multiplexed access (TDMA) for signaling in both directions and passive optical splitters for branching, thereby achieving low cost, while compromising power budget, signaling integrity, and security.
Although the art of transmitting data from a central office to a remote unit is well developed, there remain some problems inherent in this technology. One particular problem is efficiently providing data at different wavelengths to different remote units. Therefore, a need exists for a system that provides data at different wavelengths to different remote units that is less expensive and complex than those using a WDM splitter.